Papers by Author: Sang Jin Lee

Abstract: Multi-component ceramic composites consisting of two, three and four phases, based on duplex microstructures of zirconia and alumina, were fabricated by a polymer complexation route employing polyethylene glycol (PEG) as a polymeric carrier. The polymer complexation route provided porous and soft powders and they were sintered after a simple ball milling process. In this study, the microstructures and flexural strengths of the multi-component (Al2O3-ZrO2-Y2O3-SrO) ceramic composites were examined on the processing variations of mole ratio and sintering temperature. The composites showed various grain morphologies according to the sintering temperature, and flexural strength of 410 MPa was obtained in the Al2O3·ZrO2·0.5Y2O3·0.4SrO composite sintered at 1600 °C for 1 h. In particular, needle-shape grains were observed in the four-component composites sintered at 1500 °C.

Abstract: New laminate design for improved toughness in hexacelsian-alumina composite is introduced. The composite is based on crack deflection in a weak interphase in the alumina matrix and hexacelsian interphase. The strength and toughness of the laminated composite were studied both qualitatively by electronic microscopy and measuring flexure strength. The metastable hexacelsian interphases had partially microcracks to provide crack deflection in the composite, and the crack deflection noticeably proceeded along the meta-stable hexacelsian interphase. Load-deflection curve for the laminate showed improved work of fracture of 2.23 kJ/m2.

Abstract: Highly sinterable lithium titanate (Li2TiO3) powder was fabricated by an organic-inorganic solution route. Liquid-type ethylene glycol (EG) was used as an organic carrier for the metal cations. Titanium isopropoxide and lithium nitrate were dissolved in the liquid-type ethylene glycol without any precipitation. The dried precursor gel showed crystalline form at 300 °C and carbon-free Li2TiO3 was observed above 400 °C. The primary particle size of the carbon-free Li2TiO3 was about 100 nm, and the structure of the crystallized powder was porous and agglomerated. The porous powder was ball-milled and easily ground to fine particles. The powder compact was densified to 93% theoretical density (TD) at relatively low sintering temperature of 1100 °C for 2 h. The densifed Li2TiO3 showed a small grain size of 1.2 μm. As well, the grain size was notably increased at 1200 °C and above. After the over grain growth, further densification was no more observed.

Abstract: The aim of this paper was the HA and β-TCP powers were synthesized by a new wetchemical method using eggshell and phosphoric acid. The biocompatibility of synthesized natural HA, HA/β-TCP(50:50) and β-TCP derived from eggshell was compared with those of as commercial chemical powder with mesenchymal stem cells derived from human bone marrow. Development of crystalline phases of the mixtures was studied as functions of mixing ratio and temperature using X-ray diffractometer. The morphological characteristics of the calcined eggshell and synthesized powders were examined by scaning electron microscopy. The in-vitro cytotoxicity and cell attachment of sintered disks were examined using human bone marrowderived multipotent stem cells(hBMSCs). Cell response was characterized by MTT assay , Alkaline phosphatase stain and RT-PCR analysis. Pure HA was synthesized in the mixing ratio of 1:1.1 wt% at 900°C for 1h. the crystallization of HA was started at 800°C in the 1:1.1 mixing ratio, ant the HA phase was continued up to the high temperatures. In the ratio of 1:1.3 and 1:1.5 wt%, β-TCP was effectively synthesized at 900°C. In the 1:1.5 ratio, β-TCP phase was detected at 700°C, and complete crystallized β-TCP was observed above 900°C. At the higher temperature than 1000°C, the β-TCP was gradually decreased and α-TCP was observed. The HA and β-TCP disk does not exert cytotoxic effect on the hBMSCs undergoing osteoblastic differentiation. In addition, the hBMSCs are adhered on the surface of synthesized natural HA and β-TCP disk as successfully as on the culture plate or as commercial chemical HA and β-TCP disk. The hBMSCs adhered on either synthesized natural HA, β-TCP or as commercial chemical HA, β-TCP disk displays undistinguishable actin arrangement and cellular phenotypes, indicating that synthesized natural HA, β-TCP does not disrupt normal cellular responses. Analysis of differentiation of the hBMSCs cultured on culture plate, synthesized natural HA, β-TCP and as commercial chemichal HA, β-TCP disk shows that three matrices are able to support osteoblastic differentiation of the hBMSCs as accessed by alkaline phosphatase staining.

Abstract: Nano scaled mullite (3Al2O3·2SiO2) powders had been fabricated by an organic-inorganic solution technique using a polyvinyl alcohol (PVA) as an organic carrier. PVA polymer contributed to a soft and porous powder microstructure, and ball milling with the porous powder was effective in making nano-sized mullite powders. In addition, the degree of polymerization of PVA affected the crystallization behavior. The fully crystallized and ball-milled mullite powders had an average particle size of 120 nm with a specific surface area of 67.0 m2/g. In this paper, the simple solution technique and milling process for the fabrication of nano scaled mullite powders are introduced. And the effects of PVA on the crystallization behavior and powder specific surface area are also studied. The characteristics of the synthesized powders are examined by using XRD, TEM, particle size analyzer and nitrogen gas adsorption.

Abstract: TiAl and TiAlN thin films are deposited on glassy carbon and Si substrates by the pulsed cathodic arc deposition process. In our pulsed cathodic arc system, because the spatial position of plasma on the surface of the evaporation source can be controlled by pulsed arc discharge, the thickness of the TiAl and TiAlN films can be controlled at nanometer scale. Amorphous stoichiometric Ti-Al films are synthesized from one Ti-Al alloy target at room temperature by changing the number of pulses of the arc discharge.

Abstract: Several titanate powders (Al2TiO5, SrTiO3, etc.) were synthesized by an ethylene glycol solution route. Titanium isopropoxide and nitrate salts were dissolved in stoichiometric proportions in liquid-type ethylene glycol without any precipitation. The parent precursor sols were dried to porous gels, and then the gels were calcined and crystallized. All synthesized titanate powders had stable crystallization behavior at low temperature and high specific surface area after a simple ball-milling process. A three-component PZT (Pb(Zr0.52·Ti0.48)O3) powder was also synthesized successfully by the ethylene glycol method. In this study, the characteristics of the multi-component titanate powders by the ethylene glycol method are examined.

Abstract: Pure and stable YAG (Y3Al5O12) powders were synthesized by a PVA (polyvinyl alcohol) polymer solution technique. PVA was used as an organic carrier for the precursor ceramic gel. The PVA affected crystallization behavior, powder morphology, specific surface area and crystalline size of the synthesized powders. The precursor gels were crystallized to YAG at relatively a low temperature of 900 °C. The synthesized powders, which have nano-sized primary particles, were soft and porous, and the porous powders were ground to sub-micron size by a simple ball milling process. The ball-milled powders were densified to 94% relative density at 1500 °C for 1h. In this study, the characteristics of the synthesized YAG powders were examined by using X-ray diffractometer, simultaneous differential scanning calorimetry and electrophoretic light scattering spectrophotometer. And the morphologies of the powders and the densified samples were observed by scanning electron microscopy.

Abstract: Nanocomposite formation of metal-metal oxide systems by mechanical alloying (MA) has been investigated at room temperature. The systems we chose are the Fe3O4-M (M=Al, Ti), where pure metals are used as a reducing agent. It is found that nanocomposite powders in which Al2O3 and TiO2 are dispersed in a α-Fe matrix with nano-sized grains are obtained by MA of Fe3O4 with Al and Ti for 25 and 75 hours, respectively. It is suggested that the shorter MA time for the nanocomposite formation in Fe3O4-Al is due to a large negative heat associated with the chemical reduction of magnetite by aluminum. X-ray diffraction results show that the average grain size of α-Fe in Fe-TiO2 nanocomposite powders is in the range of 30 nm. From magnetic measurement, we can also obtain indirect information about the details of the solid-state reduction process during MA.